Assessing the state of union in a bone fracture

ABSTRACT

The method involves having the subject, to whose fractured limb an external fixator has been applied, undergo a specific load test, measuring the load carried by the fixator and the total load carried by the limb during the test, and determining a measure representing a comparison between the two measured loads. The test is preferably a dynamic one, and a suitable apparatus for carrying out the method is described, which includes a forceplate incorporated in a treadmill assembly to stimulate muscle activity in the limb during the test. The method and device provides an early indication as to whether or not bone healing has started, and thus can be used to provide an advance warning in cases of delayed osseous union.

This application claims benefit of international application PCT/ GB95/00313 filed Feb. 15, 1995.

The present invention relates to medical apparatus, and moreparticularly to a device and method for use in assessing the state ofunion in a bone fracture.

External fixators have been widely used in surgery especially fortreating traumatic lesions of large bones, where, after fracturereduction, the burden on the fractured bones is instead carried by afixator during the healing period. The general method involves boringpins into the healthy bone tissue on either side of the fracture andapplying a rigid support frame spanning the fracture to the protrudingends of the pins. When healing of the bone tissue is judged to becomplete the external fixator can be removed.

Being outside the body means that the fixator frame lies at a distancefrom the bone and the resulting lever arm can lead to considerable loadson the fixator frame and/or on the bone. On the other hand externalfixation has the advantage that the main components of the device areexternal to the body and therefore accessible. Strain gauge transducerscan be incorporated into external fixators for monitoring loads anddeformations over the fracture, and such biomechanical monitoringenables an assessment of fracture healing to be made, to supplementradiology and manual examination in determining the timing of theremoval of the fixator. If the supporting external fixator is removedprematurely, late angulation and refracture are common importantcomplications.

A number of methods of measuring the state of fracture healing have beendeveloped. For example, in Clinical Biomechanics, 1992, No. 7, pages75-79, `Fracture stiffness measurement in the assessment and managementof tibial fractures`, the authors J B Richardson et al describe theresults of fracture bending stiffness tests on a group of patients withtibial fractures treated with external fixators. The fracture healingmeasure proposed in that paper is a value of fracture bending stiffnessin the sagittal plane of 15 Nmdeg⁻¹ at which removal of the externalfixator and functional loading of the fracture can be prescribed. Thearticle also indicates that such a technique of fracture healingmonitoring shows no significant increase in stiffness until about eightweeks after fixation.

In Engineering in Medicine, Vol. 16, No. 4, October 1987, London (GB),pages 229-232, `The measurement of stiffness of fractures treated withexternal fixation`, J L Cunningham et al, further test equipment andprocedures are described for measuring the mechanical stiffness of suchfractures. The external fixator described in this publication is fittedwith a transducer able to measure loads (eg. torsion, bending) in anumber of different directions, and some of the results indicated thatincreased stiffness may be observed as early as five or six weeks afterfixation.

A substantial problem in healing of fractures, especially tibialfractures, is delayed osseous union. Treatment of this problem, bytechniques such as bone grafting, should be performed as early aspossible. However, conventional techniques using clinical andradiographic examination are slow to detect this problem. Biomechanicalmonitoring methods involving fracture stiffness, such as those referredto above, provide measures to determine when union is effectivelycomplete and treatment can finish, but do not give an indication as towhether or not the process of healing is properly underway, particularlyat the earliest stages of healing.

The aim of the present invention is to improve the above situation andto provide a device and method for use in assessing the state of unionin bone fractures, particularly for early detection of delayed osseousunion.

In a first aspect, the invention provides a device for use in assessingthe state of union in a bone fracture in a limb, the device comprising:

a fixator connectable across the fracture by means of a plurality ofelements extendable through respective portions of the fractured bone;

means for providing a measure of the load carried by the fixator;

means for providing a measure of the total load passed through the limbduring a specific load test; and

signal processing means for processing signals representing the fixatorload measure and the total limb load measure,

wherein means are provided to stimulate or encourage muscle activity inthe limb during the test.

In respect of the means to stimulate or encourage muscle activity in thelimb, a treadmill assembly may be provided.

The fixator load measuring means may be arranged to provide a measure ofthe fixator load in a direction substantially parallel to the long axisof the fractured bone, and for this purpose the fixator preferablyallows movement only in said long axis direction, the fixator loadmeasuring means comprising a strain gauge to detect such movement.

In an embodiment of the device, the fixator comprises two separateblocks mounted on a linear bearing, between which blocks is mounted aspring unit incorporating the strain gauge.

Additionally or alternatively, means to measure the bending moment inthe fixator may be provided.

Preferably, the total limb load measuring means comprises a forceplate,and this forceplate may form part of the treadmill assembly.

In a second aspect, the invention provides a method for use in assessingthe state of union in a bone fracture in a limb of a subject, for whichfracture a fixator has been applied, the method comprising the steps of:

subjecting the limb to a specific load test involving the application ofmuscle loading to the limb;

measuring the load carried by the fixator and the total load passedthrough the limb during the test; and

determining a measure representing a comparison between the measuredfixator load and the total limb load.

The load test may be a walking test carried out on a treadmill, thetotal limb load being determined by measuring the vertical ground forceexerted by the limb on the treadmill.

When a walking test is used, it is preferable to consider only aselected part of each step for purposes of determining the loadcomparison measure, and this selected part may be a central portion ofthe stance phase of each step.

Preferably, the test is carried out at successive intervals afterfixation and the results of successive tests are compared withprescribed results for a fracture of the type in question. In this way,monitoring the progress of the results of the test over time willprovide an early indication as to how the fracture healing isprogressing.

The comparison measure is preferably a fracture stiffness index (FSI),or its reciprocal, defined as:

    FSI=Total limb load/Fixator load.

The invention has arisen from the inventor's experimental investigationsinto the loads carried by a fixator during healing. By way of example,reference is made to the accompanying drawings which illustrate themethod and results of the experimental investigations, in which:

FIG. 1 shows the experimental set-up used;

FIG. 2 illustrates in detail the fixator device used in the experiments;

FIG. 3 shows an alternative fixator device; and

FIG. 4 illustrates the experimental results.

EXPERIMENTAL METHOD (FIG. 1)

Two similar groups of blackface ewes had midshaft tibial osteotomiesperformed. The sheep of one group, referred to henceforth as `thecontrol group`, were then externally fixed with external fixators 2 asdescribed in more detail below, whilst in the case of each of theanimals of the other group (referred to here as `the delayed-uniongroup`), the same device was fixed after the tibia had been stripped of20 mm periosteum on either side of the osteotomy and then covered by asilicone rubber sheath. In this way, the sheep of this group provided amodel of delayed osseous union, as the bone sheathing had the effect ofpreventing the blood supply from the surrounding muscles from reachingthe bone (devascularisation), and thereby delaying union of thefractured tibia.

At weekly intervals thereafter a fracture stiffness index (FSI) wasdetermined for each of the sheep in both groups. The FSI is defined asthe total load carried by the involved limb (ie. the instantaneousweight of the animal on that limb) divided by the load on the fixator,as measured during a controlled specific test, in this case a walkingtest. A specially constructed treadmill 3 was used for these tests, asillustrated in FIG. 1, consisting of an endless belt of a standardbelting material driven by an AC induction motor with a variable speedcontroller. A rectangular forceplate 4 incorporating a load transducerwas mounted horizontally below the belt at the same height as the restof the bed in order to allow measurement of the vertical component ofthe ground reaction force, representing the total load carried by thelimb. The forceplate was designed and positioned such that the rightrear hoof of an animal would normally land on it during a walking test,and the other hooves would not. A walking test was used because thisallows a considerably greater load to be applied to the broken tibiathan would otherwise be practicable, due to the muscular activityinvolved in such a test. A slow walking speed of 1 m/s was chosen forthe tests, and the system was designed so that during a test the animalwas able to slow down and stop the belt at will.

Although the treadmill used and described above was specificallyconstructed for these tests, such devices are in themselves known. Atreadmill of this general type is disclosed in International PatentApplication WO-93/06779, in this case for application to gaitcharacterisation of a subject.

For the fixator load measurement, the axial load on the fixator wasmonitored during the tests. Each external fixator 2 incorporated anin-built axial load transducer 15 to measure the fixator axialcompression (described further below). During a test, the load measuredby this transducer and that measured by the transducer incorporated inthe treadmill forceplate 4 were simultaneously recorded. The signalswere fed from the transducers to strain gauge amplifiers 5 whose outputswere fed to a microcomputer 6 for recording and analysis.

Each sheep 1 was encouraged to walk by someone standing behind it, andwhen the animal was walking steadily the test began. The signal from theforceplate transducer was sampled at 50 Hz and only recorded when itexceeded a chosen threshold of 10N, which was above the backgroundnoise, in order to define a length of time of the stance phase of eachstep. The recordings lasted between 2 and 5 minutes during which time20-150 valid steps could be recorded, valid steps being selectedaccording to defined criteria (based on shape and duration) to avoid theuse of signals recorded when the animal stopped or other such problems.The signals were automatically stored on disc by the recordingmicrocomputer 6 for later analysis.

The FSI was calculated from the recorded data for each valid step ofeach animal in each walking test in the following way. Over the middlehalf of the stance phase of each step (that is, ignoring the first andlast quarters) the vertical ground reaction force signal was divided bythe fixator axial compression signal. For each animal, the mean FSI overthis middle period was then calculated, and the values so obtained fromall the valid steps then averaged for each test, giving the values showngraphically in FIG. 4 and discussed below.

Some of the animals were killed at two weeks and the rest at six weeksin order to perform post-mortem torsional tests to measure the state ofunion of the tibial fractures directly.

The External Fixator

As FIG. 2 shows, the fixator comprises four sections, two separate pinblocks 10 (proximal) and 11 (distal), a linear bearing with twocarriages 12, and a module 13 which houses a spring 14 and an axial loadtransducer 15. The two pin blocks 10 and 11 are each provided with threebores 16 transverse to the longitudinal orientation of the fixator toreceive six fixator pins (not shown) which can be firmly clamped in thepin blocks by means of clamping bolts 17.

The proximal pin block 10 is mounted on the two linear bearing carriages12, while the lower block is bolted directly to a linear bearing track20, on which the carriages run. This bearing features preloadedre-circulating balls running within two grooves, one on either side ofthe track, providing the minimum possible play whilst ensuring lowfriction even under high load. With this device, movement is onlypossible in the longitudinal direction, which corresponds to the longaxis of the bone when the fixator is in position.

The module 13 is mounted between the pin blocks and bolted to them bymeans of flanged plates 30 and bolts 31, as indicated in FIG. 2. Betweenthe plates 30 the axial load transducer 15 and a spring block 14 arepositioned. The spring stiffness of the fixator can be varied ifrequired by replacing one spring block 14 of a known stiffness byanother spring block of a different known stiffness. For theseexperiments, a silicone spring block and an epoxy resin spacer 18 wereused, and the stiffness could be altered by varying the relativethicknesses of the two components.

The incorporation of a spring block in the fixator allows a small amountof controlled axial movement which is known to tend to encouragefracture healing. In reduction of the fracture and fitting of theexternal fixator, a small gap was left between the bone ends across thefracture site. It is to be noted that the experiments described couldalso have been carried out using rigid fixators incorporating an axialload transducer but allowing effectively no movement between the pinblocks.

A short 125 mm screened lead 32 is connected to the axial loadtransducer 15. This lead was then connected by a screened cable to astandard strain gauge amplifier for the duration of the tests.

To ensure performance, before each experiment the fixators were strippeddown, cleaned and lubricated and the pins and fixator were sterilised.The bearings were additionally periodically lubricated throughout theexperiments with a general purpose oil. In attaching the fixator, thefixator pins, which were standard 6 mm diameter Orthofix pins, werescrewed into the fractured bone at the desired positions, three oneither side of the fracture. The pins were sheathed to a diameter of 10mm outside the body for extra rigidity. The pins were then introducedinto the bores 16 and clamped in place with the clamping bolts 17.

Before each test, the clamping bolts were loosened and the fixator pinsthen tightened within the bone with a torque wrench. The clamping boltswere then tightened to firmly clamp the pins.

In the alternative embodiment of a fixator illustrated in FIG. 3,tapered bone pins 40 are shown screwed in place in the fractured bone 50and clamped in fixator bar 41 by means of pin clamps 42. The modulecomprising load transducer 43 and spring 44 is shown in detachedposition, and reference 45 indicates the linear bearings of the device.

Results (FIG. 4)

In the control group of ewes, a significant increase in FSI was detectedafter only three weeks from a mean of 0.41 (sd=0.06) at the first weekto 0.99 (sd=0.64) at the third week. This was a surprising result, assubsequent radiographs at four weeks showed advancing collars ofmineralised callus on either side of the osteotomies, but only veryminimal coalescing of these regions, suggesting very little mechanicalbridging and hence minimal load bearing capacity across the fracture.

By contrast, in the delayed-union group there was no significantincrease in the mean FSI over such a time period, the FSI value goingfrom 0.40 (sd=0.11) at the first week, 0.38 (sd=0.09) at the third weekto 0.98 (sd=1.42) at the sixth week. The results are shown graphicallyin FIG. 4, in which FSI is shown on a logarithmic axis against timeafter osteotomy in weeks. The graph shows very clearly that by a timeonly three weeks from surgery in most cases, an indication of thecommencement of bone healing can be observed by the level of FSI. Atfour weeks, there is no overlap at all between the ranges of data forthe two groups. In fact, the logarithmic scale of FIG. 4 masks thedramatic rise in FSI values. It can be seen that to begin with, thevalues for each animal are remarkably similar at around 0.4, until thefirst detectable stages of healing at which time they rapidly increase,in some cases by an order of magnitude or more.

The post mortem torsional tests performed on the osteotomies confirmedthe validity of the delayed union model. For example, of six sheep ofthe delayed union group killed at two weeks all six tibiae showed nodetectable torsional properties. Of the tibiae of the seven killed aftersix weeks, three still showed no torsional properties, whilst theremaining four measured minimal torsional stiffness. The tibiae of thecontrol group, on the other hand, were found by the torsional tests tobe at a significantly more mature stage both at two and at six weeks.

The results of these tests show that load monitoring of an externalfracture fixator, carried out in a manner according to the invention,gives strong evidence of delayed union as early as three weeks afterfixation. In practical human medical terms, such an indication can beinterpreted by a clinician as an aid in deciding whether further activeintervention to encourage union should be considered. For example, thefracture site can be stimulated or bone can be grafted to the fracturesite. In some cases an intramedullary nail can be introduced across thefracture to initiate union. The advantage of being able to take thesemeasures at the earliest possible time is that it ensures the minimumpossible total healing time. Moreover, the operation is surgically morestraightforward than would otherwise be the case as, for example, lesscallus or other material might need to be excised by a surgeon.Additionally, the monitoring of fracture healing in this way permits aquantifiable comparison of different treatments.

As mentioned previously, the test used in this monitoring method ispreferably a walking test, in order to encourage muscle loading of theinvolved bone. If the limb is being used in this way then the totalforce on the bone and fixator is made up of a body weight component anda component due to muscle loading. The walking test thus ensures thatthe bone is subjected to a force considerably higher than wouldotherwise be possible, thus making the test much more sensitive thanprevious fracture stiffness monitoring techniques, whilst maintainingthe consistency and repeatability of the results. The experimentsdescribed in this specification produced results which led to thefinding that, for a given subject, the muscle load on the bone issubstantially directly proportional to the weight component, or measuredvertical ground force. The precise role played by the muscles is notdetermined during the tests, but the above finding allows this role tobe taken into account by way of the FSI. A treadmill is therefore notessential and other specific tests involving alternative appropriatetypes of loading of the limb are envisaged.

It is to be noted that, for a completely reduced fracture, axial loadingwill give a falsely high value of fracture stiffness index. However, aspreviously mentioned, a small gap can be initially left between the boneends across the fracture, thereby providing that a valid measure ofaxial fracture stiffness can still be monitored by way of the test.

The method of the invention is equally suitable for application in thecase of a fully reduced fracture, in which case the bending moment inthe fixator frame can be used to provide the fixator load measure usedin the calculation of fracture stiffness index. In the experimentsdescribed above the bending moment in the fixator was measured by meansof an additional, suitably calibrated, strain gauge transducer attachedto the fixator frame.

A bending FSI was calculated in a similar manner to the axial FSI, thedenominator in this case being the measured fixator bending moment. Thebending FSI is clearly not a dimensionless index and its actualmagnitude will therefore be expected to vary from one subject toanother, but appropriate calibration ensures that this fact does notaffect the validity of the results. Once again the FSI values showed anoticeable increase at or about the three-week point in the case ofanimals of the control group, this discontinuity being absent in thecase of the animals of the delayed-union group. The advantage of using ameasure of bending moment in carrying out the method of the invention,apart from its applicability to well reduced fractures, is that in thecase of many types of fixator the bending moment is easier to measureaccurately than the axial load.

The results of successive tests carried out according to the method ofthe invention can be compared with prescribed results for a fracture ofthe type in question. For example, an indication can be provided whenthe results diverge from within prescribed limits, thus suggesting thatthe healing is not proceeding in a satisfactory manner.

Embodiments of the invention described and illustrated in thisdescription and in the accompanying figures are given by way of exampleonly and it is to be understood that these are not intended in any wayto limit the scope of the invention.

I claim:
 1. A system for use in assessing the state of union in a bonefracture in a limb, the system comprising:a fixator including aplurality of elements for selectively engaging respective portions ofthe fractured bone, whereby the fixator is selectively connected acrossthe fracture; means operatively coupled to the fixator for providing ameasure of the load carried by the fixator and for providing a signalrepresenting the fixator load measure; means for providing a measure ofthe total load passed through the limb during a specific load test andfor providing a signal representing the total limb load measure; andsignal processing means operatively coupled to the fixator load measuremeans and the total limb load measure means for processing signalsrepresenting the fixator load measure and the total limb load measure;wherein an apparatus is provided to stimulate or encourage muscleactivity in the limb during the test.
 2. A system according to claim 1,including a treadmill assembly.
 3. A system according to claim 1 orclaim 2, wherein the fixator load measuring means is arranged to providea measure of the fixator load in a direction substantially parallel tothe long axis of the fractured bone.
 4. A system according to claim 3,wherein the fixator allows movement only in said long axis direction andthe fixator load measuring means comprises a strain gauge to detect suchmovement.
 5. A system according to claim 4, wherein the fixatorcomprises two separate blocks, a spring unit incorporating the straingauge being mounted between said blocks.
 6. A system according to claim1, wherein the total limb load measuring means comprises a forceplate.7. A system according to claim 2, wherein the total limb load measuringmeans comprises a forceplate and wherein the forceplate forms part ofthe treadmill assembly.
 8. A system according to claim 1, includingmeans to measure the bending moment in the fixator.
 9. A method for usein assessing the state of union in a bone fracture in a limb of asubject, for which fracture a fixator has been applied, the methodcomprising the steps of:providing a fixator including a plurality ofelements for selectively engaging respective portions of the fracturedbone; connecting the fixator across the fracture; operatively couplingmeans for providing a measure of the load carried by the fixator to thefixator; providing means for providing a measure of the total loadpassed through the limb during a specific load test; operativelycoupling signal processing means to the fixator load measure means andthe total limb load measure means for processing signals representingthe fixator load measure and the total limb load measure; providing anapparatus to stimulate or encourage muscle activity in the limb duringthe test; subjecting the limb to a specific load test involving theapplication of muscle loading to the limb; measuring the load carried bythe fixator and the total load passed through the limb during the test;and determining a measure representing a comparison between the measuredfixator load and the total limb load.
 10. A method according to claim 9,wherein the test is a walking test.
 11. A method according to claim 10,wherein the test is carried out on a treadmill and the total limb loadis determined by measuring the vertical ground force exerted by the limbon the treadmill.
 12. A method according to claim 10 or claim 11,wherein only a selected part of each step of the walking test isconsidered for purposes of determining the load comparison measure. 13.A method according to claim 12, wherein the selected part is a centralportion of the stance phase of each step.
 14. A method according toclaim 9, wherein the test is carried out at successive intervals afterfixation and the results of successive tests are compared withprescribed results for a fracture of the type in question.
 15. A methodaccording to claim 9, wherein the measure of comparison is a fracturestiffness index (FSI), or its reciprocal, wherein FSI is defined as:

    FSI=Total limb load/Fixator load.